selftests/bpf: Fix dynptr/test_probe_read_user_str_dynptr test failure

When running bpf selftests with llvm18 compiler, I hit the following
test failure:

  verify_success:PASS:dynptr_success__open 0 nsec
  verify_success:PASS:bpf_object__find_program_by_name 0 nsec
  verify_success:PASS:dynptr_success__load 0 nsec
  verify_success:PASS:test_run 0 nsec
  verify_success:FAIL:err unexpected err: actual 1 != expected 0
  torvalds#91/19   dynptr/test_probe_read_user_str_dynptr:FAIL
  torvalds#91      dynptr:FAIL

I did some analysis and found that the test failure is related to
lib/strncpy_from_user.c function do_strncpy_from_user():

  ...
  byte_at_a_time:
        while (max) {
                char c;

                unsafe_get_user(c,src+res, efault);
                dst[res] = c;
                if (!c)
                        return res;
                res++;
                max--;
        }
  ...

Depending on whether the character 'c' is '\0' or not, the
return value 'res' could be different.

In prog_tests/dynptr.c, we have
  char user_data[384] = {[0 ... 382] = 'a', '\0'};
the user_data[383] is '\0'. This will cause the following
error in progs/dynptr_success.c:

  test_dynptr_probe_str_xdp:
  ...
        bpf_for(i, 0, ARRAY_SIZE(test_len)) {
                __u32 len = test_len[i];

                cnt = bpf_read_dynptr_fn(&ptr_xdp, off, len, ptr);
                if (cnt != len)
                        err = 1; <=== error happens here
  ...

In the above particular case, len is 384 and cnt is 383.

If user_data[384] is changed to
  char user_data[384] = {[0 ... 383] = 'a'};

The above error will not happen and the test will run successfully.

Cc: Mykyta Yatsenko <[email protected]>
Signed-off-by: Yonghong Song <[email protected]>

Author: Yonghong Song

tools/testing/selftests/bpf/prog_tests/dynptr.c

@@ -45,7 +45,7 @@ static struct {
 
 static void verify_success(const char *prog_name, enum test_setup_type setup_type)
 {
-	char user_data[384] = {[0 ... 382] = 'a', '\0'};
+	char user_data[384] = {[0 ... 383] = 'a'};
 	struct dynptr_success *skel;
 	struct bpf_program *prog;
 	struct bpf_link *link;